Move GCPPubSub out of contrib dir.

[n3wscott]

Stage work related to knative/eventing#1565

Release Note

NONE

/assign @Harwayne

[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-eventing-contrib-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
gcppubsub/pkg/adapter/adapter.go	Do not exist	61.0%
gcppubsub/pkg/adapter/pubsub.go	Do not exist	66.7%
gcppubsub/pkg/apis/sources/v1alpha1/gcp_pubsub_types.go	Do not exist	71.4%
gcppubsub/pkg/apis/sources/v1alpha1/gcp_pubsub_validation.go	Do not exist	87.5%
gcppubsub/pkg/apis/sources/v1alpha1/register.go	Do not exist	100.0%
gcppubsub/pkg/reconciler/gcppubsubsource.go	Do not exist	90.5%
gcppubsub/pkg/reconciler/pubsub_wrapper.go	Do not exist	66.7%
gcppubsub/pkg/reconciler/resources/receive_adapter.go	Do not exist	100.0%

[vaikas]

Sorry, I meant we should remove it from this repo to another repo, not to move it to top level. I'm sorry :(

[vaikas]

actually, this is fine :)

[vaikas]

/lgtm
/approve

[knative-prow-robot]

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